1. Field of the Invention
This invention concerns ignition systems for internal combustion engines, in general. More specifically, it relates to an improvement for a particular type of ignition system that employs high-frequency continuous-wave spark energy. The improvement relates to an aspect of the control for such an ignition system. The control involves the use of a control winding for starting and stopping the oscillation of a square wave oscillator, which produces the indicated high-frequency continuous-wave spark energy.
2. Description of the Prior Art
A highly successful ignition system has been developed which employs a single transformer, and makes use of a high-frequency continuous-wave signal that is delivered to the spark plugs. It has a controlled duration that may be determined in various manners, and it ensures a superior spark signal at each of the cylinders. Such an ignition system is exemplified by the U.S. Pat. No. 3,961,613, issued June 8, 1976. Also, there are additional patents that show and describe the same basic type of superior ignition system that is of concern here. However, it has been found that because the control winding of those ignition systems was being controlled by a transistor acting as an electronic switch, the current and/or power requirements created the need for a very expensive transistor in order to have the necessary power rating.
The aforementioned electronic control of the indicated type of ignition system made use of what may be described as a series pass transistor. It acted in series with a control winding on the above indicated single transformer which was a high voltage power type that delivered the spark signals. During the off state of the high-frequency continuous-wave spark signals, a DC current flowed through the control winding and the series pass transistor to ground. Then when a spark signal was required a high voltage oscillator was turned on by stopping the flow of the DC current through the control winding. The consequent decaying magnetic flux was sufficient to start the oscillator. Stopping the DC current flow was accomplished by turning off the series pass transistor. The oscillator would continue to run as long as the series pass transistor was off, and it would develop an AC voltage in the control winding. But, when the series pass transistor was off no current flowed in the control winding, either AC or DC.
At the end of a spark signal the oscillator would be stopped by turning on the series pass transistor. That would allow both the DC current from the battery and AC current from the oscillator action, to flow. The AC current flow would be sufficient to overload the oscillator and cause the oscillation to cease.
In a system such as just described, the starting of the oscillator reliably, required a certain amount of DC flux to be present in the transformer core when the circuit was broken. That flux is proportional to the current times the number of turns in the control winding. If the current was large, then the current drain on the battery was at a high level during the times when the oscillator was not oscillating. On the other hand, if the number of turns in the control winding was large, then a large AC voltage would be generated in this winding while the oscillator was running. Such voltage would appear at the collector of the series pass transistor. And if that voltage was too large, the breakdown voltage of the transistor would be exceeded and the transistor would fail.
In the foregoing type system, in order to stop the oscillator, it was necessary to draw enough power into the control winding circuit to reduce the loop gain of the oscillator to less than a gain of one. That required the control winding to be essentially short circuited. And since there was a high voltage present at the collector of the series pass transistor when it was turned on, a very large current would flow momentarily. Also, if the series pass transistor was capable of handling the large current surge, the oscillator would shut down. However, if the oscillator did not shut down on the first current surge, the oscillator would continue to run and cause the transistor to draw repetitive high surges of current which would soon destroy it.
Thus, it has been found that a series pass transistor in the foregoing system had to be capable of withstanding about 300-400 volts on the collector while off, and to handle current surges of about 10-50 amperes. So a transistor meeting such requirements was very expensive.
Consequently, it is an object of this invention to improve a particular ignition system that has a superior AC spark signal.
There is a U.S. Pat. to Fisher No. 4,097,770 issued June 27, 1978, that discloses a triggering circuit for a silicon controlled rectifier. However, it is applied to a capacitor discharge type of automobile ignition system, and consequently is not relevant to the applicant's invention.